This Phase I SBIR Application centers on a new class of non-??-lactam antibacterial agents that target the same biological target as the ?-lactam class of antibiotics (i.e., the Penicillin Binding Proteins, or PBPs), but with the prospect of avoiding n important determinant of cross-resistance to ?-lactams. We have identified a novel series of highly selective, non-cytotoxic small molecule lead compounds that bind to and disrupt PBP function in both gram-positive and gram-negative bacteria. Preliminary data show that this new series, exemplified by prototype lead compound PBP-539, is chemically impervious to ?-lactamase degradation, a dominant resistance determinant for all ?-lactams, and one that is growing at an alarming rate. This lead prototype shows good anti-bacterial activity against core gram positives (S. aureus and S. pneumoniae) and both wild-type and ESBL-expressing gram negatives in the Enterobacteriaceae family (E. coli and K. pneumoniae). In addition, the early leads have good aqueous solubility (>1 mg/mL), low protein binding (<50% in mouse, dog, and human serum), high selectivity vs. mammalian serine-based enzymes (IC50 vs. 6 different mammalian serine proteases > 100 ?M), and are not cytotoxic to mammalian cells (CC50 >256 ?g/mL for 3 cell lines). During the 2-year timeframe of the Application, we intend to advance the chemical optimization of this series to an Advanced Lead stage (a key milestone in drug discovery between Lead identification and Preclinical Candidate selection). This will involve: (a) improving potency levels across core pathogens (S. aureus, S. pneumoniae, ESBL-producing Enterobacteriaceae); and (b) demonstrating in vivo proof of concept efficacy in a murine model of bacterial infection. Success would trigger submission of a Phase II grant proposal wherein a second phase of Lead Optimization (Late Stage) would be performed directed at delivering a 1st Preclinical Candidate. Ultimately, it is envisioned that this new class of cell wall synthesis inhibitors may serve as a platform for a portfolio of products with different coverage spectra and modes of administration. Such new products could serve as a critical advance in the search for new treatments for serious multi-resistant bacterial infections in the nosocomial and community settings.